Non impact printing devices such as devices based on thermal mechanisms, inkjet mechanisms, and laser mechanisms, are commonly used instead of impact based printing devices. A known type of non impact printer comprises an array of minute light emitting diodes (LED's) forming a print head, for example with a row of chips. Each chip typically comprises approximately 118 to 472 elements per centimeter which is equivalent to 300 to 1200 elements per inch. The array of LED's is positioned in the printer to face a photoreceptive surface. The photoreceptive surface is charged by the printer and placed in motion relative to the LED array. The LED's are selectively enabled by an electronic circuit in the printer, responsive to data supplied to or generated by the printer. Areas exposed to light from the LEDs are discharged, producing a latent dot image of uncharged areas on the photoreceptive surface. A charged toner or ink is brought in contact with the photoreceptive surface to develop the latent image. The developed image is transferred to a hardcopy material such as paper.
LED based print heads have been used to form latent images in electrophotography.
In general, the LEDs on a chip are not exactly identical or positioned exactly, resulting in minor variations in the uniformity of the optical spot size and deviation in their position. Even microscopic differences may result in noticeable distortions in a printout, when they are repeated along a column.
As used in this document and in the claims, the term “width” is defined as the extent of a photo-surface to be exposed in a direction perpendicular to a direction of relative motion between light sources and the photo-surface. A “row” is defined as a single line of pixels or light sources that runs substantially the entire width of the photo-surface to be exposed. A line that does not run the entire width is termed a “partial row”. A column is defined as a plurality of pixels or light sources having a dimension perpendicular to a row. When a group of two or more partial rows of light sources are used to effectively form a single row of light sources, the group as a whole is referred to herein as a row.
U.S. Pat. No. 5,630,027, the disclosure of which is incorporated by reference, describes a system for splicing two partial rows of LEDs that are used to expose a photoreceptor. In this patent linear arrays of LEDs that are shorter than the width of the photoreceptor are used. Light from the arrays is focused onto the photoreceptor such that a portion of the extent of the arrays overlap. One LED from one of the arrays is used to expose each pixel along the line. However, from each row of pixels a different breakpoint between the arrays of LEDs is used, to break up any pattern that would be formed if a constant breakpoint were used This patent also describes U.S. Pat. No. 5,061,049 in which multiple partial rows of LEDs are projected onto the photoreceptor surface. As indicated in U.S. Pat. No. 5,061,049, the disclosure of which is incorporated by reference, both partial rows of LEDs can be used to expose any pixel, by providing a suitable delay between the partial rows, such that the amount of light provided to the surface is increased. U.S. Pat. No. 5,630,027 shows four partial rows. U.S. Pat. No. 5,061,049 appears to describe only two.
U.S. Pat. No. 5,757,411 also shows overlapping linear partial row arrays. However, in this patent reduced intensity of the overlapping elements is apparently used.
EP Patent publication EP 0 933 925 A2 describes a system for exposing a photographic material to multiple levels of brightness, in which multiple rows of light sources are used to expose each pixel. Each pixel is selectively exposed by a light source as the photo-material passes the particular light source. The number of sources activated for a given pixel determines the total exposure.
LED based printing devices may be limited in speed by the speed of a bus that supplies the data to the head and/or the electronic circuitry associated with the head. Furthermore, there is no backup for a LED. If a single LED fails the whole print head needs to be replaced. Another possible limitation on printing speed may be a limitation on the amount of light energy that can be supplied by the LED during a period in which it can expose a given pixel.